1. Field of the Invention
This invention relates to a device for heating transfusion fluids such as blood and drugs in solution prior to their intravenous drop-by-drop introduction to patients and, in particular, to a heating device with the above-mentioned function which is characterized in controlling the temperature of transfusion fluid to a suitable one which approximates the human body temperature, as well as in preventing the fluid from being heated beyond a certain predetermined temperature if the rate of flow of the fluid has considerably changed during drip transfusion.
2. Description of the Prior Art
Fluids for transfusion are generally stored at a temperature of 4 degree centigrade. In order to be able to safely inject the fluid into the veins of a patient, it is necessary to heat the fluid from the cold state up to the human body temperature or an approximate temperature, i.e., 37 degrees centigrade or so. There are two different manner of doing this: One is to heat the entire fluid quantity to be administrated, at one time, together with its container, and the other is to heat the fluid as it flows through a conduit connected to the fluid container. The former manner takes much time, and thus is not useful in case of emergency, while the latter manner makes it possible to heat a required amount of transfusion fluid when required. So, in most cases, the latter manner is followed.
U.S. Pat. No. 4,293,762 (issued on Oct. 6, 1981) discloses a device for heating a transfusion fluid flowing through an conduit connected to the fluid container which comprises a case having therein an electric circuit and a heater, a heating plate disposed in the front face of the case and having a groove with a winding portion, and a lid for closing the case. The electric circuit includes two thermistors. One of them measures the temperature of fluid flowing and heated through the conduit disposed in and through the groove, and operates the electric circuit to cause the heater to output intermittently for the purpose of giving a predetermined temperature to the fluid through the conduit. The other thermistor is connected to the heater for breaking the circuit if the heater has been heated beyond a predetermined temperature due to an extremely increased rate of flow of the fluid through the conduit.
However, this prior art has the following disadvantage. Usually the operational temperature of the above-second mentioned thermistor is set at a value higher than the temperature to which the fluid is to be heated (e.g., 63 degrees centigrade), so that the heater can be prevented from being heated beyond the operational temperature of the thermistor. Nevertheless it is not possible to protect the heating plate against becoming extremely hot. Then, in particular, in case of an appreciable reduction or increase of the rate of flow of the fluid, it may be overheated temporarily or partly, resulting in its coagulation or deterioration.